Reorganize template definition structure

Reorganize template definition structure

Having maintained this repo for awhile we can now see some common
patterns emerge from our yaml structures. Typically for a given job
there is a common builder anchor followed by 2 template sections, 1
for Gerrit and 1 for GitHub. Plus some common macros / anchors that
may be used by multiple job types.

This patch reorganizes the structure to treat a set of
anchor-gerrit-github job definitions as a single job definition. This
allows us to keep the job template close to their anchor definition so
that looking up what the main anchor does will not cause as much back
scroll.

Additionally this patch refactors the confusingly named verify_merge
anchors to a more understandable 'common' name.

Change-Id: I95eb4609274ec21b50aef059b03ac4598894f671
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>